Recently, as the technique of solar electricity generation has been widely used, large number of solar cells using crystal silicon, amorphous based semiconductor and the like have come to be manufactured and installed on roofs in the shape of modules, or sold as structures integrated with the building material such as roof or wall of the buildings.
A thin film based solar cell module using an amorphous based semiconductor or the like allows mass production, and allows formation as a simple integrated structure on a substrate. Therefore, such solar cell module is advantageous in view of cost, and hence particularly attracting attention these days.
An outline of the manufacturing process of the thin film based solar cell module having the integrated structure on substrate is as follows. First, a transparent electrode layer of SnO.sub.2 or the like is formed by thermal CVD method or the like, on a glass substrate, and patterned by laser processing or the like. Thereafter, a photo semiconductor layer is formed by plasma CVD method or the like thereon, and similarly patterned by laser processing or the like. Thereafter, a metal film, or a multi-layered film including a transparent electrode and metal as a back electrode layer is formed further thereon by vapor deposition or the like, and finally patterned, whereby a plurality of photo semiconductor devices are formed interconnected on one glass substrate. Simple interconnection is provided for taking out terminals, and thereafter, the back surface is sealed and protected by a filler and a back surface protective sheet, and thus a solar cell is completed. By putting the solar cell into a support member such as a frame formed of aluminum with sealing material formed of butyl rubber or the like interposed, a solar cell module can be fabricated in a simple manner.
FIG. 7 is a cross sectional view representing a schematic structure of one example of the conventional thin film based solar cell module having integrated structure on substrate.
Referring to FIG. 7, the solar cell module includes a solar cell 1, and an aluminum frame 21 supporting solar cell 1, and solar cell 1 is fitted in aluminum frame 21 with sealing material 4 of butyl rubber interposed.
FIG. 8 is a partial cross sectional view showing, in enlargement, solar cell 1 of the solar cell module shown in FIG. 7.
Referring to FIG. 8, solar cell 1 includes a glass substrate 10, and a plurality of photo semiconductor devices 50 formed of a transparent electrode layer 20, a photo semiconductor layer 30 and a back electrode layer 40 formed successively on glass substrate 10. A device forming surface (back surface) side of glass substrate 10 is sealed by a filler 60 and protected by a back surface protective sheet 70.
FIG. 10 is a cross sectional view representing a schematic structure of another example of the conventional thin film based solar cell module having the integrated structure on substrate. The solar cell module is designed assuming that it is to be installed on the roof of a building.
Referring to FIG. 10, the solar cell module includes a solar cell 1, a support base 5 supporting solar cell 1 and a pressing member 23. Support base 5 is formed of wood, and pressing member 23 is formed of aluminum. Further, support base 5 is adapted to be attachable on an iron plate (not shown) spread on the roof of a building. Further, between solar cell 1 and support base 5, a cushioning material 33 formed of polyurethane, for example, is interposed. The structure of solar cell 1 is completely the same as that shown in FIG. 8, and therefore, description thereof is not repeated.
Of the solar cell module structured as described above, not only stable electricity generating characteristic but also weather resistance against environment elements such as ultra violet ray, acid rain, heat and the like and strength against mechanical damage including cracks are important requirements on reliability, as the solar cell is installed at not easily accessible positions.
Though breakdown voltage of the conventional solar cell module measured between a power lead out terminal and the frame satisfied the JIS standard immediately after fabrication, a phenomenon of significant decrease in the breakdown voltage was observed in some samples, when measured after acceleration test at high temperature and high moisture.
Therefore, the inventors studied the cause of decrease in the breakdown voltage, and found that the conventional thin film based (substrate integrated type) solar cell module suffers from the following problem, as assembly structure of the supporting member supporting the solar cell, such as the frame or the pressing member, is the same as that for the crystal based solar cell module.
FIG. 9 is an illustration representing the problem experienced in the conventional substrate integrated type thin film based solar cell module.
Referring to FIG. 9, in the conventional solar cell module, when solar cell 1 is assembled with frame 21, sealing member 4 formed of butyl rubber is interposed, which sealing member 4 of butyl rubber floats because of exposure for a long period of time. It was found that this causes movement of solar cell 1 in frame 21, resulting in contact between solar cell 1 and frame 21. Especially when the solar cell module is installed on an inclined roof, if sealing member 4 experiences plastic deformation, solar cell 1 easily moves downward because of the weight of solar cell 1.
In the thin film based solar cell 1 having such a structure as shown in FIG. 8, sometimes transparent electrode layer 20 may extend around third surfaces or to the light entering surface side opposite to the photo semiconductor device forming surface of glass substrate 10. In such a case, insulated state cannot be maintained if glass substrate 10 simply contacts frame 21. Particularly, in the solar cell module having such a structure as shown in FIG. 10, glass substrate 10 is in contact with pressing member 23 formed of aluminum, and it is difficult to maintain insulated state if there is a wraparound of transparent electrode 20.
Accordingly, the inventors considered measurement for insulation, by removing transparent electrode 20 formed extending around the periphery or an end portion of glass substrate 10 to attain similar condition as a crystal based solar cell module.
When an SnO.sub.2 transparent electrode layer is to be formed on the glass substrate by the method of thermal CVD or the like, however, it is very difficult from the essential nature of gas phase reaction, to mask only the peripheral portion of the glass substrate so as to prevent formation of the SnO.sub.2 layer.
A method of removing the SnO.sub.2 layer at the peripheral portion of the glass substrate using a grinder or a chamfering apparatus together with the glass substrate after formation of the transparent electrode layer was also studied. This method, however, increased processing cost, and further, raised a problem that generated particles caused pin holes in the next step of forming photo semiconductor layer. Therefore, it was found that the method could not be used.
On the other hand, the inventors noted the problem of improving functional reliability of the solar cell module, in addition to the problem of decrease in breakdown voltage described above.
More specifically, in most cases, a fragile material such as glass or transparent ceramics is used as the glass substrate (hard transparent plate) on the side of the light entering surface of the solar cell. The reason for this is that it is far superior in weather resistance to transparent resin. As represented by the fact that the term "brittle" is easily associated with "glass", as is well known, glass and ceramics are disadvantageous in that they are fragile. One counter measure is use of a reinforced glass, for example. Considering the condition where the solar cell is installed outdoors for generating electricity, however, it should be noted that the temperature of the solar cell attains to high temperature of 70.degree. C. or higher. The solar cell is adapted to have such a structure that has extremely high light absorptance to improve photo electric conversion efficiency, and therefore temperature increase is quite abrupt as compared with the structures of the aluminum frame or the roof. In an extreme case, there may be a temperature difference close to 50.degree. C. from the periphery. As is well known, the frame and the like are formed of metal and have high thermal conductivity, and it was found by the inventors that a large temperature gradient generates in the (hard transparent plate) having small coefficient of thermal conductivity in operation. This is a factor significantly impairing mechanical reliability of the solar cell. It is well known that glass brittles when locally heated or cooled abruptly. The conventional solar cell module had the problem that possibility of such phenomenon caused in operation of the solar cell module was extremely high.
Particularly, the problem of mechanical reliability has been pointed out in the solar cell module having such a structure as shown in FIG. 10.
An object of the present invention is to solve the above described problems and to provide a solar cell module having superior breakdown voltage reliability and/or mechanical reliability.